1. Technical Field
The present invention relates to a lane keeping control system that causes a vehicle to travel along a predetermined target course by driving an electric power steering motor.
2. Related Art
Recently, in order to reduce traffic accidents and driver's load, techniques concerning various lane keeping control systems that assist and control a steering operation to cause a vehicle to travel along a predetermined target course have been developed and proposed. For instance, Japanese Unexamined Patent Application Publication (JP-A) No. 2007-125959 discloses a steering system for a vehicle. Specifically, the steering system for the vehicle receives data on a target course and has a driver model that estimates a driver steering input necessary for the vehicle to travel along a target course on the basis of a predetermined driver steering characteristic. The steering system determines a steering assistance amount by adding a feedforward steering assistance amount, which is obtained by multiplying an estimated driver steering input by a feedforward gain, to a feedback steering assistance amount, which is obtained by multiplying an amount of positional deviation between the vehicle traveling course and the target course by a feedback gain, and by weighting the feedforward gain to be larger than the feedback gain when the positional deviation is large and weighting the feedback gain to be larger than the feedforward gain when the positional deviation is small.
There are various driver steering conditions when a driver overrides the lane keeping control system for the vehicle. Therefore, if the driver overrides the system using a vehicle steering device disclosed in JP-A No. 2007-125959, in which the vehicle steering device performs feedforward control and feedback control while changing the weighting therebetween on the basis of an amount of positional deviation between the target course and the vehicle traveling course, the lane keeping control system might not satisfactorily response to the driver's steering operation. That is, it is required that a vehicle steering device perform precise lane keeping control in the case where the driver substantially entrusts control of the vehicle to the vehicle steering device, however, when the driver applies a steering input to the steering device, it is required that the lane keeping control continue without excessively interfering with a driver's steering intention. If the lane keeping control system controls against the driver's steering intention, there is a possibility that the driver may feel discomfort.
There are also various characteristics of feedback control. If a distance from a present vehicle position to a forward attention point (a point at which a deviation is calculated) is set to be short, the system promptly controls the vehicle to follow the target course by observing a position immediately in front of the vehicle. In such a case, tracking performance for the target course becomes good, but controllability of a target steering angle and vehicle behavior becomes worse in sensitivity and stability since the system mainly responds to a current lateral deviation alone. In contrast, if the distance from the present vehicle position to the forward attention point is set to be long, the vehicle has only to return to the target course before the vehicle reaches the far forward attention point. Although the tracking performance for the target course becomes worse, controllability of a target steering angle and vehicle behavior becomes good in stability since the system can perform a predictive control in consideration of the orientation of the vehicle with respect to the target course, and rate of change (yaw rate) of the orientation as well as the lateral deviation from the target course. As described above, when the driving assistance control is performed in the forward attention model, it is necessary to suitably set the feedback control so as to balance between the tracking performance to the target course and the stability of the vehicle behavior.